1. Field of the Invention
The present invention relates generally to a method for measuring nuclear magnetic resonance properties of an earth formation traversed by a borehole, and more particularly, to a method for eliminating any ringing, such as magnetoacoustic ringing, and DC offset, during a nuclear magnetic resonance measurement.
2. Background of the Art
A variety of techniques are utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, the resistivity, porosity and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling, which is referred to as measurement-while-drilling (MWD) or logging-while-drilling (LWD).
One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things, porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the liquids in the geological formations surrounding the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as T1) and transverse relaxation time (generally referred to as T2) of the geological formations can be measured. From such measurements, porosity, permeability and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
The NMR instrument also typically includes an antenna, positioned near the magnet and shaped so that a pulse of radio frequency (RF) power conducted through the antenna induces an RF magnetic field in the earth formation. The RF magnetic field is generally orthogonal to the field applied by the magnet. This RF pulse, typically called a 90 degree pulse, has a duration and amplitude predetermined so that the spin axes of the hydrogen nuclei generally align themselves perpendicularly both to the orthogonal magnetic field induced by the RF pulse and to the magnetic field applied by the magnet. After the 90 degree pulse ends, the nuclear magnetic moments of the hydrogen nuclei gradually xe2x80x9crelaxxe2x80x9d or return to their original alignment with the magnet""s field. The amount of time taken for this relaxation, referred to as T1, is related to petrophysical properties of interest of the earth formation.
After the 90 degree pulse ends, the antenna is typically electrically connected to a receiver, which detects and measures voltages induced in the antenna by precessional rotation of the spin axes of the hydrogen nuclei. The precessional rotation generates RF energy at a frequency proportional to the strength of the magnetic field applied by the magnet, this frequency being referred to as the Larmor frequency. The constant of proportionality for the Larmor frequency is known as the gyromagnetic ratio xcex30. The gyromagnetic ratio is unique for each different chemical elemental isotope. The spin axes of the hydrogen nuclei gradually xe2x80x9cdephasexe2x80x9d because of inhomogeneities in the magnet""s field and because of differences in the chemical and magnetic environment within the earth formation. Dephasing results in a rapid decrease in the magnitude of the voltages induced in the antenna. The rapid decrease in the induced voltage is referred to as the free induction decay (FID). The rate of FID is typically referred to by the notation T2*. The FID decay rate consists of a first component referred to as xe2x80x9ctrue T2xe2x80x9d, which is due to internal nuclear environmental effects, and a second component resulting from microscopic differences in magnetic field and inhomogeneities in the earth formation. The effects of the second component can be substantially removed by a process referred to as spin-echo measurement.
One problem with analysis of NMR measurements is that the signal detected by the antenna includes a parasitic, spurious ringing that interferes with the measurement of spin-echoes. One source of the spurious signal is electromagnetic generation of ultrasonic standing waves in metal. The induced RF current within the skin depth of the metal interacts with the lattice in a static magnetic field through the Lorenz force and the coherent ultrasonic wave propagates into the metal to set up a standing wave. A reciprocal mechanism converts the acoustic energy, in the presence of the static field, to an oscillating magnetic field which is picked up by the antenna as a spurious, ringing signal.
Different types of magnetoacoustic interaction may produce a parasitic signal in the NMR antenna. Antenna wiring and other metal parts of the NMR logging tool can be affected by the static magnetic field and the RF field generated by the antenna. If the antenna is located within the strongest part of the magnet""s field, when RF pulses are applied to the antenna, acoustic waves are generated in the antenna and the antenna sustains a series of damped mechanical oscillations in a process known to those skilled in the art as magnetoacoustic ringing. This ringing can induce large voltages in the antenna which are superimposed with the measurement of the voltages induced by the spin-echoes.
Another source of magnetoacoustic interaction is magnetorestrictive ringing which is typically caused when nonconductive magnetic materials, such as magnetic ferrite, are used in the antenna. If this magnetic material is located within the strong part of the RF field, application of RF pulses will generate acoustic waves in the magnet. The magnet will experience a series of damped mechanical oscillations upon cessation of the RF pulse. Magnetorestrictive ringing can also induce large voltages in the antenna which are superimposed with the measurement of the voltages induced by the spin-echoes.
One approach to reduce the effects of ringing has been to design the hardware to minimize the interaction between the electromagnetic fields and the materials in the device. For example U.S. Pat. No. 5,712,566 issued to Taicher et al. discloses a device in which the permanent magnet composed of a hard, ferrite magnet material that is formed into an annular cylinder having a circular hole parallel to the longitudinal axis of the apparatus. One or more receiver coils are arranged about the exterior surface of the magnet. An RF transmitting coil is located in the magnet hole where the static magnetic field is zero. The transmitting coil windings are formed around a soft ferrite rod. Thus, magnetoacoustic coil ringing is reduced by the configuration of the transmitting coil. Magnetorestrictive ringing of the magnet is reduced because the radial dependence of the RF field strength is relatively small due to use of the longitudinal dipole antenna with the ferrite rod. Further, magnetorestrictive ringing is reduced because the receiver coil substantially removes coupling of the receiver coil with parasitic magnetic flux due to the inverse effect of magnetostriction.
Another commonly used approach to reduce the effect of ringing is to use a so-called phase-alternated-pulse sequence. Such a sequence is often implemented as
RFAxc2x1xxe2x88x92xcfx84xe2x88x92nxc2x7(RFByxe2x88x92xcfx84xe2x88x92echoxe2x88x92xcfx84)xe2x88x92TWxe2x80x83xe2x80x83(1)
where RFAxc2x1x is an A pulse, usually 90xc2x0 tipping pulse and RFB is a pulse, usually a 180xc2x0 refocusing pulse. The xc2x1 phase of RFA is applied alternately in order to identify and eliminate systematic noises, such as ringing and DC offset through subsequent processing. By subtracting the echoes in the xe2x88x92 sequence from the pulses in the adjoining + sequence, the ringing due to the 180xc2x0 is suppressed.
PCT publication WO 98/43064 of Prammer addresses the problem of ringing caused by the excitation pulse. A dual frequency acquisition is carried out with phase alternation, the separation between the two frequencies being one fourth of the reciprocal of the delay time in acquisition between the excitation pulse and the first refocusing pulse. Averaging of the two measurements then attenuates the effect of the ringing due to the excitation and refocusing pulses.
A drawback to the averaging of phase alternated data sequence is the requirement to combine two pulse sequence cycles. Measurements made by an NMR logging tool in this manner are therefore subjected to degradation in the vertical resolution due to the logging speed, wait time between each pulse sequence, and the data acquisition time. In addition, the logging tool moves along the longitudinal axis of the borehole between each of the measurements.
The problem with logging speed is exacerbated in multifrequency NMR measurements. A pulse sequence for an eight frequency logging operation may be denoted by
CPMG(f1, TE1, RFA+, n1)xe2x88x92t1xe2x88x92CPMG(f2, TE2, RFA+, n1)xe2x88x92t2xe2x88x92 . . . 
CPMG(f8, TE8, RFA+, n8)xe2x88x92t8xe2x88x92CPMG(f1, TEi, RFAxe2x88x92, n1)xe2x88x92t1xe2x88x92CPMG(f2, TE2, RFAxe2x88x92, n2)xe2x88x92t2 . . . CPMG(f8, RFAxe2x88x92, n8)xe2x88x92t8xe2x80x83xe2x80x83(2)
where fi, TEi and ni are the frequency, interecho time and number of echoes for the i-th CPMG echo train. The CPMG pairs that only differ in the RFA phases are 8 sequences apart. Unless the logging speed is slowed down significantly, the two sensed volumes will be spatially separate and distinct the resolution of the tool is impaired.
The Sun patent teaches a method for suppressing ringing in which the acquired data consists of N normal echoes followed by M so-called xe2x80x9cspoiledxe2x80x9d echoes. The spoiled echoes do not have NMR signals from the formation and consist of noise only. By using the estimate of the noise signal, a signal may be recovered in which the noise has been attenuated. This method overcomes the disadvantages of PAP averaging discussed above; however, a substantial amount of time and energy is devoted to acquisition of spoiled echoes. As a result of this, either logging time has to be increased with an accompanying increase in the heating of the RF probe, which subsequently changes ringing, and increases power consumption duty cycle.
The present invention is a method of improving the resolution of NMR signals received from a formation surrounding a borehole. Any pulsed NMR tool in which a magnet arrangement is used to generate a static magnetic field having a substantially uniform field strength in a region of the formation surrounding the borehole, and in which an RF antenna is used to produce pulsed RF fields substantially orthogonal to the static field in the region of examination may be used. The tipping pulses in successive samples have alternating polarities. Differences between successive samples of the in-phase and quadrature component signals are determined and averaged over a sample window to give an estimate of the in-phase and quadrature component noise. These are indicative of the DC offset and the ringing noise in the received signals. The estimated in-phase and quadrature noise values are then used to correct the raw data. The sample window may be of fixed or variable length or may be recursive. In one embodiment of the invention, the noise estimate may be made variable to account for RF heating of the magnet and the antenna.